The present invention relates generally to electronics and more specifically to a device and method for supplying power at a set voltage over a wide range of current values for parametric testing and simultaneously providing the ability to measure the current driving the load (device) being tested with three-figure (i.e., xc2x10.1%) accuracy.
When performing parametric testing on a device such as a transistor or on an entire integrated circuit (IC), it is frequently necessary to provide power at an accurate voltage over a wide range of load current (many orders of magnitude) and to accurately measure this current. Also, it is necessary to provide a quiet voltage (i.e., no voltage spikes). For example, in parametric characterization it may be desirable to provide a constant voltage over a load current range of from 1 ampere down to 1 nanoampere (10xe2x88x929 amperes) while accurately measuring the load current over this range.
The various types of equipment currently available for providing a constant voltage during parametric testing all suffer from deficiencies which impede testing. One method for providing power during parametric testing is a conventional power supply. Typical power supplies do not provide, however, either high voltage resolution or accurate current measurement ability. Standard power supplies do provide current read back using an analog-to-digital converter (ADC) to measure the current. An ADC is an electronic circuit that receives a magnitude-scaled analog voltage and generates a binary-coded number proportional to the analog input, which is delivered to an interface subsystem to a digital computer. The ADC is usually twelve-to-sixteen bits and is set for full-scale at maximum current. This configuration yields current measurement resolution in the milliampere range, which is inadequate for smaller load currents.
Digital multi-meters (DMMs) typically do not provide the broad range of current measurement necessary for parametric testing. Most DMMs only provide three orders of magnitude (103) of current range and not the nine orders of magnitude (109) necessary for parametric characterization. Other DMMs have a tendency to go into xe2x80x9cvoltage compliancexe2x80x9d as they range upscale, losing the ability to maintain a constant voltage at the load. The current goes out-of-range with a loss of control over the output voltage, and builds up a voltage burden at the output measuring hundreds of millivolts.
The deficiencies of the conventional equipment and methods for supplying a constant, quiet voltage and measuring load current during parametric testing show that a need still exists for improvement. To overcome the shortcomings of the conventional equipment, a new circuit is provided for supplying a constant voltage and measuring load current. It is an object of the present invention to provide a circuit for supplying a constant voltage and accurately measuring load current during parametric testing. It is another object of the present invention to provide a quiet voltage environment.
To achieve these and other objects, and in view of its purposes, the present invention provides a circuit for driving a load voltage to the same value as an input voltage over a wide range of currents. The circuit keeps the voltage quiet and stable. The circuit also accurately measures the current drawn by the load.
The circuit of the present invention incorporates a sense driver and a bypass driver. The load voltage can be driven by the sense driver, the bypass driver, or both, by way of switches (relays) at the driver outputs (e.g., the feedback loops). The bypass driver comprises a high-current operational amplifier (op amp) capable of driving a load voltage at up to 2 amperes and a feedback loop. The sense driver differs from the bypass driver in that it has a high-impedance op amp connected in series with a high-current op amp. The high-current op amp has a direct feedback loop and operates as a voltage follower. The sense driver further differs from the bypass driver in that it includes a series of sense resistors, any one of which can be selected (i.e., connected into the high-impedance op amp feedback loop). A particular sense resistor can be selected by closing its corresponding switch (relay) and opening the corresponding switches (relays) for each of the other sense resistors.
The sense resistors are used to create a voltage drop whose magnitude is dependant upon the load current. The load current flows through the selected sense resistor, generating a voltage drop equal to the load current times the resistance of the selected sense resistor. Because the resistance of the selected sense resistor is known to within xc2x10.1%, the load current is calculated by dividing the measured voltage drop by the known resistance.
An appropriate sense resistor must be chosen for the magnitude of the load current being measured. If the resistance is too low, the voltage drop will be too small to measure accurately. If the resistance is too high, the sense driver will not be able to provide a high enough voltage output to supply the voltage drop (i.e., the sense driver will saturate), and the load voltage will no longer equal the input voltage. A microcontroller and voltage comparator circuit can be used to choose the appropriate sense resistor to avoid either of these undesirable conditions.
When switching sense resistors in and out by way of the switches (relays), momentary conditions may occur for which the output of the sense driver generates noise. This noise should not be coupled to the load, as it may cause damage. This condition can be avoided by first switching xe2x80x9cinxe2x80x9d the bypass driver, by connecting the bypass driver to the load by closing a bypass relay, and then switching xe2x80x9coutxe2x80x9d the sense driver, by disconnecting the sense driver from the load by opening a sense relay. In this way, the sense resistors can be switched while the sense driver is not connected to the load. The sense driver is switched back xe2x80x9cinxe2x80x9d and the bypass driver is switched back xe2x80x9coutxe2x80x9d for load current measurements.
The present invention provides considerable improvement over the conventional devices available for providing a constant voltage during parametric testing. A constant, accurate, quiet voltage is driven to a load, while providing an autoranging current sense point. The sensed current can be made accurate over many orders of magnitude by providing a bank of sense resistors, each an order of magnitude apart from the previous resistor (e.g., 1 ohm, 10 ohms, 100 ohms, etc.).
The present invention also encompasses a method of providing a constant voltage to a load over a range of currents and for accurately measuring the current drawn the load. The method comprises a number of steps. Specifically, the method provides a programming voltage to the inputs of a sense driver and a bypass driver. The bypass driver consists of a high-current op amp acting as a voltage follower. The sense driver comprises a high-input impedance op amp with a feedback loop, which drives its output voltage to its input voltage plus the voltage drop in its feedback loop. The sense driver feedback loop comprises a high-current op amp, which acts as a voltage follower, providing an output voltage equal to its input voltage at a current required to provide such output voltage to the load, and a sense resistor selected from a series of sense resistors having different resistance values.
After providing a programming voltage to the inputs of the sense driver and the bypass driver, the method selects the appropriate sense resistor such that its voltage drop at the load current can be accurately measured without saturating the sense driver. The method then connects the bypass driver, the sense driver, or both to a load. Finally, the method measures the voltage drop across the sense resistor which is in the sense driver feedback loop to determine load current.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.